Peer Review of the Stability Analysis of the Gypsum Stack at the Widows Creek Fossil Plant

TVA OIG retained Marshall Miller & Associates, Inc. (Marshall Miller) to conduct a peer review of the Stantec Consulting Services, Inc. (Stantec) report on geotechnical exploration and slope stability for the gypsum stack at the Widows Creek Fossil Plant. Marshall Miller believes that Stantec's evaluations of the Widows Creek gypsum stack provide a reasonable assessment of the margin of safety associated with the evaluated conditions, which indicates that the facility is not in danger of imminent failure.Marshall Miller did find that additional analyses and corresponding documentation was needed in order to assess the overall factor of safety of the stack in the midterm and longterm. Marshall Miller also found that Stantec used a model that was 20 feet lower than the final height of the stack which does not reflect the final conditions of the pile. Additionally, Marshall Miller found that Stantec did not (1) perform adequate testing to support reliance on historical data and shear strength characterization of some materials, (2) calculate and document the exit gradient and factors of safety against piping for the 5-year build-out configuration, and (3) perform sufficient investigation of the clay foundation soils.To address this report, TVA management had Stantec review and respond to the findings of this report. Marshall Miller reviewed Stantec's response and concluded that the additional information provided adequately addressed the concerns and recommendations identified in the report.